The invention relates to a method and apparatus to mark a document using a combination between guilloches and watermarking, and a method and apparatus to read the previous combination.
Security is one of the highest challenges for “official” documents today. A credential needs to take into account the following factors: physical and data security. Controlled by persons with minimum equipment and motivation, the credential must have sufficient observable security features to allow quick visual verification. Sensitive data stored on the credential need to be encrypted and signed by the issuing authority. The role of a secured system is to improve confidence in verifying the identity of individuals seeking access to physical or virtual locations.
A wide variety of systems require reliable personal recognition schemes to either confirm or determine the identity of an individual requesting their services. All the innovations in this field which are simultaneously relevant for the application context, user friendly, real time and low cost are worth investigating.
Two major tools are used to solve this: guilloches and watermarking.
Fine line graphics, known as guilloche patterns are a traditional anti-copy feature found on most printed documents of any value. As a guilloche consists of a fixed pattern, one can scan a real picture, determine the guilloche pattern and add this pattern on its own picture. After that the attack consists of printing the picture on a “blank” card, with the false personal data. As the picture matches with its cardholder and looks like a valid one, the way to detect the counterfeiting is based on other countermeasures. The below picture illustrates this feature. This countermeasure protects the picture against modification but not against substitution.
Watermarking can be defined as a process that aims at intelligently incorporating a watermark signal (a pseudo random noise) to a host signal in such a way that it can be recovered from the signal mixture while being imperceptible for the human eye. An efficient watermarking scheme has therefore to combine imperceptibility and robustness. To assure robustness, the watermark information has to be incorporated in a redundant way in the host data so that it can be recovered even in the presence of only a small amount of data. Robustness is also dependant on the strength of the insertion. The stronger insertion the better the resistance to different signal attacks. The requirements of robustness naturally conflict with imperceptibility. The trade-off between imperceptibility and robustness is managed by the insertion scheme itself and it is also controlled by the mark effect or degradation on the image. This control is commonly assured by the use of a psycho visual mask aiming at modeling the behavior of Human Visual System (HVS). The watermark signal is often designed in the spatial domain but also in a transform domain like the full-image DCT (Discrete Cosine Transform), the block wise DCT domain, DFT (Discrete Fourier Transform) domain, wavelet domain and sometimes in the fractal domain. The signal embedding is generally done by addition/multiplication or by the use of histograms, mostly in the luminance channel alone, one color channel and less often to several color channels. Some recent works have showed the interest to use the various color spaces in order to minimize the image deterioration after the watermarking stage.
We are interested in the capability of watermarking techniques to decode pictures printed on plastic medium. Several industrial applications related to smart cards could be imagined to improve secure authentication. Watermarking techniques constitute an interesting additional ingredient to cryptography and can be advantageous combined with biometric technologies. In this area, images such as identity pictures have a particular interest, and the study is limited to this image class. In terms of robustness, the specifications for the domain are critically the robustness to printing and scanning attacks as completely integrated into the process and the resistance of the watermark to the card durability (usage along the whole life time), say dithering, scratches, color fading. In terms of perceptibility, the human perception is the unique criterion. It should be also noted that the detection scheme has to be very practical, highly efficient and fast to constitute an acceptable solution.
The main problem with geometrical attacks (global and local distortions) remains the image synchronization before the watermark detection. The mark is decodable only if in a certain way the native numerical format image can be retrieved. There have been a number of synchronization schemes (so-called second watermarking generation techniques) designed to deal with geometrical attacks including the printing and scan process, described in F. Lefebre, D. Gueluy, D. Delannay and B. Macq, “A Print and Scan Optimized Watermarking Scheme.” The main known are an exhaustive random search over the space containing the set of acceptable attack parameters, the design of insensitive domains and template insertions, for example described in the following reference: S. Pereira and T. Pun, “An Iterative Template Matching Algorithm Using the Chirp-Z Transform for Digital Image Watermarking.”
Today, geometric attacks still pose a great obstacle to the acceptance of image watermarking for practical commercial applications. If many watermarking approaches in the numerical world (internet) are operational, progress is needed to have the same in the digital world (involving hardware transformations for the image).
The watermark has to be retrieved via a simple scanner and some computations. The condition is that the scanned image can be synchronized to the numerical one.
There is no solution capable of matching with all requirements with good efficiency while being fast, secure and imperceptible. The card context is particularly problematic as print/scan attacks seriously affect the original signal and make more difficult the image synchronization.